I have the chance to add a 2nd 7990 to my system, I am current mining WDC on a single 7990 which gives me around 30 coins a day, it was higher but the difficulty went up last night. The seller of the card wants 1BT for it, I bought my first card from him for 0.5 BT when the 7990 was a lot cheaper but it appears the have gone up in price. I would pay 1.2BT back.
Since you already have some income, just wait until you mine enough and save the interest. Otherwise, have collateral ready.